A particle therapy is a cancer treatment method which comprises accelerating charged particles such as protons, carbon ions or the like up to about a several hundreds mega-electron volt by use of an instrument such as an accelerator or the like, followed by radiating them to a patient to thereby impart a dose to a tumor in his/her body. At this time, for the tumor, it is important to form a dose distribution that is as close as possible to a dose distribution ordered by a doctor, namely, a target distribution. In many cases, the target distribution is such a distribution in which the dose in the tumor is as uniform as possible and the dose outside the tumor is as lower as possible than that in the tumor.
In general, when the particle beam accelerated by an accelerator is radiated to an object (examples of which include a human body), the three-dimensional dose distribution in the object has a feature of having a maximum dose peak at one certain point. The maximum dose peak is referred to as Bragg peak. In addition, when the distribution has the maximum dose peak at one point in a three-dimensional space, the position of such a peak is defined as “irradiation position” of that particle beam. In order to three-dimensionally form the target distribution using the particle beam with such a peak structure described above, some kind of ingenuity is required.
As one method for forming the target distribution, there is a scanning irradiation method. In order to employ this method, firstly, such a feature is used that arbitrarily deflects, using electromagnets, etc., the particle beam in two directions perpendicular to a Z-direction that is a traveling direction of the particle beam, namely, X and Y-directions. Further, such a feature is required that adjusts the energy of the particles to thereby arbitrarily adjust in the Z-direction, the position at which the Bragg peak is formed. Generally, a particle beam generation-transportation apparatus that performs transportation and interruption of the particle beam is provided with an accelerator for accelerating the particle beam, and the accelerator also has an energy adjusting function. Then, upon setting a plurality of irradiation positions (referred to also as spots) in the tumor, the particle beam is radiated using the above two features, sequentially to the respective irradiation positions. The balance among the doses to be individually imparted to the respective irradiation positions has been adjusted and determined beforehand, so that the target distribution is formed as the result of totaling the respective dose distributions applied to the respective irradiation positions.
According to the scanning irradiation method, because various uncertain factors exist in actual irradiation, there is a possibility that, although the target distribution must be obtained on a calculation basis, the dose distribution actually obtained is not matched to the target distribution. The uncertain factors includes, for example, a temporal change in the particle beam quantity, a temporal change or a hysteresis in the magnetic field of a scanning electromagnet, a sensitivity variation of a dose monitor, a signal delay and/or a noise of a control device, and the like. It is thought that, due to influence by them, an actual dose distribution possibly becomes different from the calculated values.
In order to eliminate the uncertainties, such an operation is generally performed in which, after the preparation of plan for a particle beam therapy but before the radiation of the beam to a patient, beam radiation is executed to a phantom (a substitute for the patient) in a condition as equal as possible to that of the plan, so that a value of absolute dose (absolute dose value) and a dose distribution therein are measured and confirmed whether they are accommodated to the plan. This operation is referred to as a patient QA (Quality Assurance). As the phantom, water filled in a water tank is generally used in many cases, so that the dose is measured using a dose measuring device placed in the water. In view of the purpose of the patient QA, it is desired to confirm not only the absolute dose value at the center of the tumor but also the dose distribution therearound, and therefore, it is desired to measure the doses at multiple measuring points.